Official from Treyarch, here's the official announcement for Black Ops Map Pack 3, entitled Annihilation.The map pack will be available exclusively for Xbox 360 on June 28th.

Just like other map packs, it contains 4 multiplayer maps, as well as 1 new zombie map.

Multiplayer

Hangar 18- Medium sized map, many paths focusing on a central hangar

Drive-In- Similar style to Nuketown, small, wide open middle area

Silo- Based on a missile silo, very large, lots of flank routes

Hazard- Inspired by Cliffside from WaW, turned into a golf course, focused on long distance combat

Zombies

Shangri-La- Very rocky, bridges, waterfalls, booby traps and rail cars to move around, monkeys return and carry power ups in some way, they may change like a random drop or steal them, can't know for sure yet

At this moment, it's only known that it will begin on the first of July. I can only assume, just like previous events, it will commence at 10 am PDT, but until an official announcement is made, we won't add to any speculation.